In many cases remote-controlled camera-vehicles are used to inspect inaccessible hollow spaces respectively for small hollow spaces endoscopes. Since more inexpensive and more efficient image-processing systems have become available, inspection systems have increasingly been equipped with image-processing systems in order, on the one hand, to assist the operator in visually examining the hollow space and, on the other hand, for (semi) automatically measuring the hollow spaces. As the primary aim largely determining the setup of the optical system (camera and illumination) is to assist the operator, the conventional devices on the market are illuminated with constant, unstructured light.
Completely, three-dimensional measurement of the inspected hollow spaces by means of camera images would require, as is known, either illumination with structured light or a second camera (stereo vision system). Furthermore, in order to achieve desired measurement accuracy, the known processes require that the components be spaced a minimum distance apart perpendicular to the inspection direction. Besides interfering with the operator's visual inspection, the use of known 3D-optical measurement procedures is usually out of the question solely because of the needed room.
In many inspection vehicles, the cameras are located on a pan-and tilt-head. The orientation of the axis of the camera occurs by rotation about the axis of the camera and about an axis running perpendicular thereto. Contrary to the usually employed orientation of the human eye by means of two rotations of the head about axes running perpendicular to the mean axis of the eye, a combination of these camera rotations ultimately yields an image of the inspection site turned about the horizon. According to the state of the art (printed German patent DE 30 19 339 C1) this rotation can be compensated by a counter rotation of the sensor element in the camera.
The following problems are encountered. In order to also permit measurement of the depth of the hollow space with one of the conventional inspection systems, as mentioned above, either a stereo-vision system or an additional source of structured light that can be switched on is disposed on the endoscope or camera vehicle, because a more or less large volume of the object space is imaged in the image plane of a lens due to the depth of sharpness of the image. An object-plane-cutting volume in the form of a truncated pyramid is assigned to each image element (pixel). Therefore, without any additional measurements, using solely a camera permits only very inaccurate measurement of the dimensions. There is no calibration of the detected structures with the imaging ratio.
A section of an image in the X-Z-plane (cf. FIG. 1 and equation {1}) makes this more apparent. Point P with the coordinates (X,Z) is imaged with a lens of focal length f onto an image point B with the coordinates (x,z). The imaging equations (taking into account image reversal by means of a suitable selection of x, z measurement coordinates) yields: X=x (Z/f-1) {1}, i.e. without knowledge of the object distance Z, the distance X of the point P from the optical axis cannot be determined.
Simple distance sensing (by way of example, using proximity detectors) can in some circumstances only occur for plane structures which are situated in a plane lying parallel to the image plane. For other objects, the distance of the single image points usually is determined by means of light section procedures or stereo cameras. These procedures are based on an assessment of the parallax of two optical systems (2 cameras or a structured light source and a camera). FIG. 2 shows the simplest example for demonstrating the principle of the light section procedure, the image of an object point illuminated by a laser beam. For the illuminated object point then applies, in addition to the imaging condition {1}, that it is cut by the illumination beam path. The laser beam intersects the optical axis at point (0,a) at an angle of w. Observation of the (X-Z) plane yielded by the optical axis and the laser beam suffices. For imaging using the lens, imaging condition {1} applies and the intersection of the object point with the illuminating laser beam yields: X=Z.multidot.tan(w)-b {2} or X=(Z-a) tan(w) {3}. In the case of the known light section procedures, the intersection point of the illuminating pattern with the principle plane of the lens is used as the reference point (b,0). Then the coordinates (X,Z) of the point P are yielded by the x-coordinate measured in the image plane, the beam angle w and the known focal distance f according to: EQU X=x(f.multidot.tan (w)-b/(x-f.multidot.tan (w)) {4}
and EQU Z=f.multidot.(x-b)/(x-f.multidot.tan (w)) {5}.
Usually it is not sufficient to only measure one point in the projected plane. Therefore, usually a line or a light structure directed to the measured object is projected. In systems according to the state of the art, the structure projector is located at a distance b from the camera. For applications in which only very compact measurement systems can be utilized, such as, by way of illustration, probes for examining pipes, in the case of the known light section systems the structure projector cannot be attached in the center. As the following plane case shows in a simple manner, this system has considerable drawbacks, in particular in examining cylindrical hollow spaces or in inspecting pipes. In this simple instance, the structure projector emits two laser beams at an angle of w=.+-.wl to the optical axis. FIG. 3 shows the setup. The beam courses and the imaging condition yield the equations {6} and {7} for calculating the coordinates (X,Z) of the light section points from the values of the x-coordinates measured in the image plane: EQU X=b/2+(f.multidot.tan (w).multidot.(x+b))/(x-f.multidot.tan (w)){6}
and EQU Z=f.multidot.(x+b)/(x-f.multidot.tan (w)) {7}:
If the to-be-measured nominal width region of the pipe or the shape and size of the to-be-measured hollow spaces is not very restricted, so that illumination with an adapted pattern (respectively optical axes of illumination and camera that are slanted toward each other) can be carried out, diagonal sections in the pipe or hollow space are measured (cf. FIG. 3). Consequently the side lying closest to the structure projector is measured with great accuracy (as the measuring points are not far from the camera), whereas the opposite side of the pipe, in which the measuring points are situated at much greater distance from the camera, is measured with less measurement accuracy. Frequently the extreme situation occurs in which parts of the light section lie beyond the zone of sharp focus of the image, i.e. they cannot be measured at all.
The measurement errors .sigma..sub.x of the x-coordinate (in the image plane) result in the measurement errors .sigma..sub.X and .sigma..sub.Z of the object coordinates X,Z given in the equations {8} and {9}: EQU .sigma..sub.x =.vertline.(f.multidot.tan (w).multidot.(b+f.multidot.tan (w)).multidot..sigma..sub.x /(x-f.multidot.tan (w)).sup.2 .vertline.{8}
and EQU .sigma..sub.z =.vertline.f.multidot.(b+f.multidot.tan (w)).multidot..sigma..sub.z /(x-f.multidot.tan (w)).sup.2 .vertline.{9}.
As the calculation of a typical course of an error of the Z-coordinate determination shows (cf. FIG. 4A), the precision of the Z-coordinate measurements in the left (broken lines) and in the right (uninterrupted line) beam path varies. Moreover, the course of measurement accuracy of the X-coordinate determination (cf. FIG. 4B, bottom), shows that with measurement systems of this type, the greatest measurement accuracy is achieved directly in front of the camera and the structure projector. The measurement accuracy in the regions not directly in front of the camera is considerably lower.
However, exactly in these outside regions lie the regions (.vertline.X.vertline.&gt;b/2) that are of interest in the inspection of hollow spaces such as pipes or inspection with endoscopes, whereas the regions in which the standard light section procedures provide the greatest measurement accuracy partially permit no section with the structured light at all (due to the geometry of the objects to be measured). Therefore, with these procedures only relatively inexact measurements can be carried out in the pipes or similar hollow spaces.
Moreover, when examining pipes with these measurement procedures, there are relatively great differences in intensity in the projected light section, and the calculation of the coordinates of the object is relatively complicated. Illuminating the pipe with a conical light structure in the system shown in FIG. 3 results in, by way of illustration, the equations {10} to {12} for calculating the coordinates X,Y,Z (for comparison see the calculation for an invented system shown in the following equations {13} and {14}): ##EQU1##
In the known systems, both the optical systems are disposed side by side and the optical axes of the systems have at least one oblique angle to this distance. In order to achieve the desired measurement accuracy, it is absolutely necessary to maintain a minimum distance between the components of the system, i.e. an extension of the systems in the direction perpendicular to the inspection direction. Accordingly, these systems can only rarely be utilized for inspecting the interior of objects having little light width (pipes, vessels, small hollow spaces, etc.).
In the three-dimensional measurement of hollow space geometry, there are different problems for both systems (stereo-vision system, camera and structured illumination), especially if modification of the rotation position of the camera image is compensated according to the state of the art.
For a system comprising a camera and a structured illumination, resolution accuracy of the individual coordinates is limited by the distance between the camera and the structured illumination. In order to ensure as simple as possible operation of the apparatus, the camera usually is disposed in the center. In this way the distance between the camera and the structure projector (which limits measurement accuracy) is limited to half of the maximum possible value (diameter of the inspection system), i.e. accuracy is additionally limited. Furthermore, when hollow spaces with curved boundaries are inspected with such a system, due to the source point of the illuminating pattern being located outside the axis, there are variations in pattern between the illuminating pattern and the pattern visible on the wall of the hollow space, as well as between these two and the projected image. For this reason, in order to determine the coordinates of the pertinent structures of the object, complicated calculations of the coordinate transformations and form transformations between the illuminating structure, the structure projected on the object, and the structure seen with the camera are necessary. The position of the distance between the camera and the light source in the space are taken into account in these structure transformations. Furthermore, this distance causes the projected pattern to shift on the camera image, the size of which depends on the distance and the angle of the inspection system to the wall of the hollow space. The known method of simplifying the calculation of object coordinates from a camera image is illumination with a pattern adapted to the geometry of the object to be measured. It cannot be used with these procedures due to the distance and angle-dependent shift of the projection of this pattern.
If, in addition, a system for compensating the angle between the image of the camera and the horizon is utilized, the rotation of the image of the camera and the illuminating structure (i.e. the compensation angle) has to still be taken into account in the calculation of the structure transformation.
On the other hand, in a stereo vision system, it has to be taken into account that the position of the camera going into the calculation of the depth data changes spatially due to the rotation of the pan-and tilt-head. Calculation complexity in determining the object coordinates continues to increase if the images of the camera are equipped according to the state of the art with a compensation of the image position in relation to the horizon.